Electric switch contact



April 3, 1951 A. PALME ETAL ELECTRIC SWITCH CONTACT Filed Feb. 24, 1947 o. C M m R GOLD COATING Uw Km m Lma. ,Lw @rum .t uH. A vhr nti .m e C CIH Patented Apr. 3, 1951 Mass., assignorsrto General Electric Company, a corporation of New York Application February 24, 1947, Serial No. 730,240

3 Claims. l

This invention relates to electric switch cone tacts and more particularly to improvements in tap changing switches which are adapted to operate submerged in hot mineral oil. A typical tap changing switch of the non-arcing duty ratio adjuster type is described in Patent No. 2,255,501, granted September 9, 1941, on an application of L. F. Blume and assigned to the present assignee. Such a switch is used for changing the ratio of a transformer and it is usually mounted in the transformer tank below the surface of the insulating and cooling liquid with which the tank is usually substantially filled. In many cases, this liquid is high quality reiined mineral oil. The contacts of the switch are usuali?,l made of silver or are silver plated because such materialhas long been considered one of the best for this purpose. However, long experience with such ratio adjuster as shown with the silver contacts in time became coated with a black scale which even- `tual1y peels oir the surface and is replaced by a new layer of scale. This has been found to be caused by traces of sulphur in the mineral oil which combine with the silver to form silver sulphide. The silver sulphide ilakes which are thus formed have been known to cause short circuits or arcovers of the switch by forming a conducting path between the xed conacts.

The obvious solution to this problem would be to eliminate the sulphur in the mineral oil but investigation has proved that this is impractical because it would be prohibitively expensive.

Another solution is to nd a suitablehcont'act material which will not be affected by the sulphur and will not form sulphides or the like which will cause deleterious eiects. Such material must be immune to tarnish in hot oil, that is to Say it must be immune to sulphurizi'ng, oxidizing or saponiiying. It must also have good electrical conductivity. Furthermore, its ohmic resistance and therefore its watt loss in contact with its companion member must be steady over a long period of years. It must have good mechanical wearing properties so as to withstand repeated heavy pressures of 40 to 75 lbs. with sliding or rubbing motion with its companion member.

In accordance with this invention, it has been discovered that the combination of gold and rhodium as the two engaging contact surfaces is a highly satisfactory combination from all standpoints.

An object of the invention is to provide improvements in switch contacts.

Another object of the invention is to provide zoo- 166) Another object of the invention is to improve the operation of tap changing switches.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. l

In the drawing, Fig. l is a side elevation view partly in phantom of a transformerv provided with' a ratio adjuster switch; Fig. 2 is a side elevation view of the switch shown in Fig. 1; Fig. 3 is a sectional View taken on line 33 of Fig. 2; Fig. 4 is an enlarged view of a pair of abutting contacts of the switch taken in the plane of Fig. 3, and Fig. 5 is a circuit diagram showing how the switch contacts are connected to a winding.

Referring now to the drawing and more particularly to Fig. 1, there is shown a conventional transformer tank I provided at itstop with high voltage bushings 2 whose inner ends are connected to a transformer winding 3 on a magnetic core 4. Taps 5 brought out from the winding 3 are connected to a ratio adjuster switch 5. The tank I is substantially filled with mineral oil up to the level indicated by 1. This mineral oil dure ing operation of the transformer is raised to a temperature substantially above that of the ambient temperature, the temperature rise being typically 50-70" C. The switch S is operated from outside the tank by an operatingshaft 8. The

details of the switch 6 are shown in Figs. 2 and 3. `Referring now to these gures, the switch comprises a pair of end members 9 of insulation between which extend a plurality of parallel contact rods or bars lil. Adjacent pairs of these rods are interconnected selectively by a bridging Contact member I I in the shape of a wedge. Any desirednumber of wedges may be used, there ac tually being two of these wedge members, one above the other, as shown in Fig. 2.

The wedges II are carried on rods I2 and springs I3 are provided for forcing the wedges between the adjacent -pairs of iixed contact rods. The switch is operated by a crank arm or member I4 on the main shaft 8. The crank lf3 is embraced by a pair of members I5 and I6 which are held together by screws. The member I5 is provided with a notch Il for engaging the various contact rods Il! during the operation of the switch.

The operation of the switch is as follows: Assume that shaft 8 is rotated clockwise as viewed from above. This will cause the crank I4 to rotate clockwise as viewed in Fig. 3, thus causing the assembly which is carried by the crank to pivot about the point of contact of the wedge II with the rods I0 and thereby cause the right hand notched end of the member I5 to move upwardly as viewed in Fig. 3. When the shaft has made about one-fourth of a revolution, the notch II is about in line with the rod I which in Fig. 3 occupies approximately the position of the numeral 2 on the face of a clock. Further rotation of the shaft 8 causes the crank I5 to back the wedge out from between the contact rods l0 to move the notch I'l into embracing relation With the previously mentioned rod Ii). Further rotation of the shaft Will then cause the wedge assembly to pivot about the notch Il and finally, when the shaft has made almost one complete revolution, the wedge will be driven -between the next lower-most pair of adjacent contact rods. This completes one cycle of the operation and it will be seen that for a little less than one complete revolution of the operating shaft 8 in either direction, the wedge Will be moved in the opposite direction from between one pair of adjacent contact rods to the next pair of adjacent contact rods.

In Fig. 3, the heavy shading of the contact surfaces indicates the gold and rhodium coatings. This is shown enlarged in Fig. 4. The gold coating is on the fixed Contact rods ID and the rhodium coating is on the Wedge I I. One reason for this is that rhodium is substantially more expensive than gold and as there is considerably more contact surface on all the fixed rods I (I than on the Wedge II, it is more economical to use rhodium on the Wedge than on the rods. The use of gold on both the wedge and the rods has been tried but the gold is so soft it produces a galling action and the contact surfaces do not Wear properly. Rhodium to rhodium also does not operate satisfactorily from a mechanical wear standpoint and, of course, it is also very much more expensive than the rhodium-gold combination described above.

The at present preferred way of applying the coatings is by electroplating. Only the actual contact surface need be plated during the plating operation. The test of the surfaces may be masked in a Well known manner.

Fig. illustrates one Iway in which the contact rods Ill may be connected to taps 5 of the winding 3. This winding may be either the primary or secondary Winding of the transformer.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the invention and therefore it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Latters Patent of the United States:

1. In an electric vswitch for operation submerged in hot mineral oil containing traces of sulphur, a plurality of insulated fixed contact members normally subjected to a potential difference, and a movable contact member for selectively and slidably engaging said fixed contact members at pressures of to 70 pounds, the surfaces of said fixed contact members which are engaged by said movable contact member having a gold coating, the surface of said movab1e contact member which engages said fixed contact members having a rhodium coating.

2. In combination, heat evolving electrical apparatus having a mineral oil filled enclosing casing, said oil containing traces of sulphur, a control switch for said apparatus, said switch being submerged in said oil, said switch having a plurality of relatively fixed contact members the surfaces of which have va gold coating, and a movable contact member the surface of which has a rhodium coating for slidably engaging the contact surfaces of said relatively fixed contacts at pressure of 40 to 70 pounds, said fixed contacts being connected respectively to different poten- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,105,399 Cadell July 28, 1914 1,248,621 Cooper Dec. 4, 1917 1,990,277 Feussner et al 1 Feb. 5, 1935 2,247,755 Hensel et al. July 1, 1941 2,255,591 Blume Sept. 9, 1941 2,379,641 Keitel July 3, 1945 FOREIGN PATENTS Number Country Date 395,896 Great Britain July 27, 1933 

